Brother, I mean no disrespect, but this is just drag and drop. I've developed dozens of games and even have one attached to this itch.io account. I'm really not sure how else to explain this.
When he updated the game in the past, he would upload the entire game again as a packaged .exe launchable file. This is no different from his Steam update, regardless of the engine update. It's still a packaged .exe version of the game. He can simply take this folder and drag and drop it into itch.io for players to download and play.
Again, I mean no disrespect, but you seem to think the process is far more complicated than it actually is. If it were a console port, then yes, you would need to "port" it. But that's not the case here. It's just an .exe file that he's packaged and uploaded to Steam, but hasn't yet uploaded to itch.io.
Edit: I do think it's important you're aware that this is a super easy process before commenting on other peoples post about it.
Edit Edit: Heck i'll throw in an example. If you were to make a game in unreal engine. You have the packaged game in unreal. People download it on itch.io. You then decide "Actually I want to remake it in unity". You remake the game in unity. You can simply just upload those unity launch files in your "next update" on itch.io. It has no bearing what engine you're using. As a user you're simply just downloading the files the creator is uploading. All itch.io is, is essentially a file sharing platform. You're just given someone a place to download the content you're uploading.